Adhesive subdivisions intrinsic to the epithelial somites

Developing somites express two subtypes of classic cadherin adhesion receptors, N-cadherin and cadherin-11 (cad11). To investigate the role of these adhesion molecules in somite morphogenesis, we analyzed the somites of mice whose N-cadherin and cad11 genes were disrupted. The epithelial somites of N-cadherin null mutant mice were fragmented as reported, whereas those of cad11(-/-) mice showed no structural anomaly. In mice double homozygous for N-cadherin and cad11 mutation, however, somites were further fragmented into smaller clusters than in the N-cadherin-deficient mice, suggesting that these two cadherins cooperate in the maintenance of epithelial somites. Despite the disorganization of epithelial structures, dorsoventral polarity markers were expressed in their correct patterns in all of these mutant somites. Uncx4.1, whose expression is localized only in the caudal region of each somite, was also expressed in a normal pattern in the mutant somites. However, the staining for Uncx4.1 revealed that, in the N-cadherin mutants, each somite tended to be cleaved at the border between the Uncx4. 1-positive and -negative regions and that the cleaved subunits maintained the clustered state, often exhibiting epithelioid morphology. This separation of the rostral and caudal regions was observed as soon as the epithelial somites had been formed. In the N-cadherin/cad11 double-homozygous mutants, this tendency was also observed, although each half of the somite further disintegrated into randomly arranged cell clusters. These results suggest that cells of the rostral and caudal regions of each epithelial somite have an activity to aggregate independently or separate from one another and that one role of N-cadherin and cad11 is to connect the two halves into a single unit.